1. Field of the Invention
This invention relates generally to tape drives and more particularly to a system of optimally controlling read/write operations under various sets of operation conditions.
2. Discussion of the Prior Art
In a modern electronic computer system there are a number of ways of recording binary information on a magnetic surface. One commonly used method is to record bits of information on magnetic tape. Here, elementary units of magnetization of two polarities are impressed on the magnetic tape in a pattern representing either a zero or a one. There are various patterns or codes which can be used. One popular encoding system is the PE system (phase encoding). This system consists generally of a positive current transition at the bit cell center for a "one" and a negative transition for a "zero" (a bit cell is herein defined as one interval along an information track when that track is divided into several equal lengths; a bit cell may also be regarded as time periods as the track moves beneath the recording head.) With this system, despite the requisite two flux changes per bit, greater bit density is possible since the random sequence of one's and zero's and data produces wide frequency bands in NRZ type of recording (to be hereinafter described), but only about octave bandwidth for the double-pulse technique. Typically, for example, NRZ techniques may have packing densities of 800 bits per inch and bit rates of 120,000 per second, whereas phase modulation techniques reliablly generate bit packing densities up to 1600 bits per inch and bit rates of 300,000 bits per second.
Another very commonly used system of magnetic recording on a track in a magnetic medium is NRZ, or non-return-to-zero Variations of this system are the NRZ-M (non-return-to-zero-Mark), NRZ-I (non-return-to-zero-inverse), and NRZ-C (non-return-to-zero-change). In the NRZ system, generally, the current direction in the magnetic recording head is unimportant; what is important is that the current shifts from one level to another for a one so as to cause a flux to saturate at the opposite level of saturation. Hence, a one would be represented by the current in the magnetic heads switching from +I.sub.M to -I.sub.M or from -I.sub.M to +I.sub.M ; whereas a zero is represented by no shift. Note that in this system only one flux change per bit is required resulting in a high pulse packing density, but the system is not self-clocking, therefore a clock track must be provided along with the data tracks, or must be provided from the data (i.e. at least one bit in each frame).
It can therefore be seen that NRZ recording identifies a digit by means of a flux change, a change from one state to another normally representing a one, and no-change representing a zero; whereas in PE encoding there is a flux change which contains information in the center of the digit interval. With PE encoding we have the greatest frequency when a string of digits are the same and the least frequency when the ones and zeros alternate. Hence, with different modes of recording we have different densities of flux reversals; with PE recording we have a higher reversal rate than with NRZ recording. Since the recovered signal is proportional to the time rate-of-change of flux, the frequency of the recovered signal will be different in accordance with the type of recording used. Consequently, an amplifier which would be suitable to give optimum amplification for one signal would be unsuitable for the other.
Furthermore, as the magnetic tape recording art progressed several manufacturers developed and marketed improved chromium-dioxide tape generally referred to as high-resolution tape. Chromium-dioxide (high-resolution tape) has a higher "coercive force*" than the standard iron-oxide tape, and it also provides higher reading resolution; consequently, it can be run at higher density. Therefore, it frequently is desirous to utilize the high-resolution tape although the industry standard is the iron-oxide low-resolution tape. Once again, however, the properties of the two tapes are so different that optimum writing, for example, on high-resolution tape is achieved by utilizing a higher write current; consequently, one utilizing high-resolution tape would desire for optimum results, a lower read amplifier gain in order to present the same amplitude signal at the tape control unit. Similarly different tapes must be read differently for optimum performance. Once again it becomes obvious that different write and read circuits are necessary to achieve optimum results when utilizing the high-resolution tapes as opposed to using the low-resolution tapes. FNT *"Coercive force" is herein defined as the amount of magnetomotive force required to change the state of the flux on the magnetic material.
Some prior art techniques for selectively activating separate recording channels on magnetic media are to be found in the following patents: (A) J. H. Gerding -- Means to Selectively Activate Separate Recording Channels -- U.S. Pat. No. 3,426,338; (B) E. A. Aron -- Track Selection Logic For Magnetic Storage Drum -- U.S. Pat. No. 3,350,700; (C) S. O. Sampson -- Track Selection Control Means for Magnetic Signal Recording and Reproducing Systems -- U.S. Pat. No. 3,315,041; (D) T. W. Holland -- Means to Utilize a Conductive Strip on a Magnetic Tape as an Indexing Device -- U.S. Pat. No. 3,376,564; (E) J. D. Meng -- Electro-Optical Apparatus and Circuit for Sensing Reflective Areas or Appertures in Tape -- U.S. Pat. No. 3,290,509. The first three of these patents pertain to track selection; whereas in the last two patents, a reflective patch is affixed on the magnetic tape and sensed to indicate when the end of the tape is approaching. Whereas these devices function to accomplish a specific task they do not appear capable of performing the various selection tasks required by the impact of advancing tape-recording technology.
Accordingly, it is a primary object of the present invention to provide an improved system for control of read/write on magnetic tape.
It is a more specific object of the invention to provide tape detection means for responding to pre-selected conditions and/or requirements, to selectively activate electronic circuits either singly or in combination to provide optimum read/write operation for the set of conditions that are prevailing.
It is still a further specific object of the invention to provide improved circuits for control of read/write on magnetic tape.
It is still another more specific object of the invention to provide a read amplifier gain switch by logic signal.
It is yet another more specific object of the invention to provide an improved tape drive system that can utilize either high-resolution tape or low-resolution tape.